Modified cellulose ester fibers



Patented Sept. 3, 1946 PATENT OFFICE.

MODIFIED CELLULOSE ESTER FIBERS Robert E. Fothergill, Julian W. Hill, and Alfred A. Johnson, Wilmington, Del., asslgnors to E. I.

du Pontde Nemours & Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application October 6, 1943, Serial No. 505,216

The invention relates to modified cellulose organic acid ester fibers. More particularly this invention relates to a process for producing modified cellulose acetate fibers which are substantially-insoluble inorganic solvents, and to the novel products of this process.

Cellulose acetate fibers have a number of properties which make them eminently suitable for use in fabricating various typesof fabrics or textiles, and they have found wide application in this field. However, they suffer from the disadvantage of being soluble in or seriously softened by a number of common Organic solvents. For this reason, fabrics containing cellulose acetate fibers are subject to damage from these solvents. Processes have been described for making cellulose derivatives insoluble in organic solvents by modification with bifunctional reagents such as hexamethylene diisocyanate, s-(bis-methoxymethyl) urea, etc. However, fibers modified with these reagents are generally too brittle to be used in all applications.

An object of this invention. therefore, is to provide modified cellulose organic acid ester fibers which are non-brittle and substantially insoluble in organic solvents. A further object is to produce modified cellulose acetate fibers substantially insoluble in organic solvents and having the de-' sirable characteristics of unmodified cellulose acetatefibers. more clearly-appear hereinafter.

These objects are realized by our invention which, stated in general terms, comprises incorporating in a spinning solution of an organic acid ester of cellulose a small amount of mineral acid and a polysilicic acid partially esterified with a monohydric alcohol of from 1 to 6 carbon atoms, spinning the resulting solution by the conventional dry or evaporative technique, and thereafter subjecting the fibers thus formed to heat treatment whereby to impart the desired insolubility. Preferably the polysilicic acid ester is one having a relatively low degree of esteriflcation v and low molecular weight, g

By the term "relatively low degree of esterification, we mean to define esters wherein the ratio of ester groups to silicon atoms is lessthan 0.6 to 1.0.

By esters of low molecular weightis meant those contained in a solution of polysilicic ester which has been aged at 25 C. less than. threequarters of the time required to produce gelation of the solution.

For purposes of convenience the invention will be further described with particular reference to 17 Claims. (c1. 1847.5)

The above and other objects will 3 cellulose acetate. It willbeunderstood, however, I

that the invention is not so'limited but applies as well to the production of organic solvent-in-' soluble fibers from any other filament-forming organic acid ester of cellulose.

The cellulose acetate used for the preparation of modified yarn of this invention may be any of the commercially. available grades which are suitable for dry-spinning into yarn. A cellulose ace-. tate containing 54.5% combined acetic acid is especially suitable.

The partially esterified polysilicic acids useful for purposes of this invention may be prepared by processes such as those described in Kirk,

Serial No. 439,549 filed April 18, 1942, now Patent No. 2,395,880, and in Iler and Kirk, Serial No- 439,548 filed April 18, 1942. Detailed descriptions of procedures for preparing preferred typ s of polysilicic acid esters are given below. Throughout the specification and claims parts and percentages are by weight unles otherwise indicated.

Procedure A.An aqueous solution of relatively low molecular weight polysilicic acid is prepared by adding 900 parts of a 15.5% solution of sodium silicate (SiO2:NazO=3.25:1 by weight) to 860 parts of a vigorously stirred solutionof 7.3% sulfuric acid over a period of 4-10 minutes. To the resulting solution (pH 1.7-1.8) 300 parts of tert.-

butyl alcohol is added and'the mixture stirred for about 10 minutes. At the end of this time 450 parts of sodium chloride is added, and stirring continued for about 5 minutes or until the salt is substantially all dissolved. The upper, tert.-butyl alcohol layer which separates when the mixture is allowed to stand for 45 minutes at room temperature, is separatedfrom the aqueous lowerlayer I and centrifuged for- 15 minutes. A small layer of water forms on the bottom of the centrifuge bottle, and a clear tert.-butyl alcohol layer forms on top with an intermediate layer of gel particles.

The clear, tert.-butyl alcohol layer, amounting to about 284 parts, is removed and immediately cooled to a temperature of 05 C; to decrease 55 of a high flash naphtha having a boiling range of 150-200 C. The concentrated acetone solution of butyl acid polysilicate which separates as the lower layer contains about 34% of SiO: and is drawn off after exactly minutes and immediately diluted with more acetone to form a solution of 5-20% S102 as butyl acid polysilicate. The dilute acetone'solution is filtered to remove a small amount of salt which is present. This contains a small amount of free sulfuric acid. A solution contains about 0.45-0.60% sulfuric acid, while a 5% solution contains about 0.11- 0.15% sulfuric acid. This amount of sulfuric acid has a deleterious effect on the yarn properties of cellulose acetate containing it as a modifier. However, if the sulfuric acid is completely removed, the stability of the butyl acid polysilicate solution is too poor for the solution to be used in the preparation of cellulose acetate yarn. -It has been found that the minimum free acidity of an acetone solution (5% SiOz) of this type of modifier, which is considered to be stable enough to be used in making a cellulose acetate spinning solution, is about 0.0150.02% sulfuric acid. An acetone solution of the desired free acidity is obtained as follows: One hundred parts of an acetone solution containing 4.7% SiOz and 0.147% sulfuric acid is stirred with 1 part of barium carbonate until the free acidity is reduced to 0.0175- 0.02%. This requires from to 2 hours. The

solution is then filtered.- If the free sulfuric acid content of the solution should bereduced to below 0.015% by this treatment, the calculated quantity of sulfuric acid is added to the filtered solution to bring the free acidity back to 0.015-0.02%. For best results, the acetone solution should be kept cold and used as soon as possible after its preparation, as the polysilicic acid ester gradually polymerizes on standing.

0.01 N hydrochloric acid, 6 parts of water, and 120 parts of absolute ethyl alcohol previously cooled to 8 C. This solution contains 1 mole of water for each mole of ethyl silicate. After thorough mixing the solution is maintained at 0 5" C.

until ready to be used in the preparation of a cellulose acetate spinning solution.

The preferred procedure for the preparation of a polysilicic acid ester-modified cellulose acetate yarn is as follows: A spinning solution is prepared by mixing cellulose acetate with a suflicient quantity of an acetone solution of a partially esterified polysilicic acid of relatively low degree of esterification and low molecular weight (pre- 7 pared as described above) to forni' a solution of filtered and spun in regular cellulose acetate dry spinning equipment under ordinary spinning conditions. The resulting yarn is heated for 1 to 2 hours at a temperature of 160-170 C. After this heat treatment, the modified cellulose acetate yarn is substantially'unafiected by immersion in hot C.) tetrachloroethane for 15 minutes and has a sticking point 15-20 0. higher than that of unmodified cellulose acetate, as determined by a test which is described later.

Procedure B.Another process for the preparation of a butyl acid polysilicate which is effective in reducing the tetrachloroethane solubility of cellulose acetate is as follows: To 1,760 parts of an aqueous solution of relatively low molecular weight polysilicic acid prepared as described above in procedure A are added 196 parts of tributyl phosphate and 460 parts of sodiumchloride. The mixture is stirred for one hour and then is allowed to stand for one hour, at room temperature. The upper, tributyl phosphate layer is separated and dried over anhydrous sodium sulfate. The yield of clear tributyl phosphate solution of butyl acid polysilicate is 125 parts. To obtain a solution of butyl-acid polysilicate suitable for addition to a cellulose acetate spinning solution, one volume of this tributyl phosphate solution is thoroughly mixed with one volume of methanol and 2.5 volumes of benzene during one or two minutes and then allowed to stand at room temperature for 12-14 minutes. At the end of this time the mixture is separated into two layers, the lower of which is a concentrated solution'of butyl acid polysilicate containing about 54% SiOz. This lower layer is immediately diluted with suflicient acetone to make a solution containing 20% 'SiOz.

The re- -sulting solution contains about 0.06% sulfuric acid and can be used directly in the preparation of a cellulose acetate spinning solution.

The preparation of a partially esterified alkylpolysilicate of low molecular weight and relatively high degree of esterification by the partial hydrolysis of a tetra-alky-l silicate is illustrated by the following example.

Procedure C.--One hundred four parts of tetraethyl silicate is added to a solution of 3 parts of The following examples further illustrate the practice of this invention.

Example I This example illustrates the preparation of a polysilicic acid ester-modified cellulose acetate yarn by use of a butyl acid polysilicate of low degree of esterification and relatively low molecular weight prepared by the tributyl phosphate extraction process as described in Procedure B above.

To 1,942 parts of a filtered acetone solution containing 510 parts of cellulose acetate. containing 54.5% combined acetic acid is added 337 parts of an acetone-methanol (4-1) solution of a low molecular weight, lowly esterified butyl acid polysilicate (prepared'by the tributyl phosphate extraction procedure) containing 21.6% SiOz. The solution, which contains 14.3 parts SiOz per 100 parts of cellulose acetate, is thoroughly mixed by tumbling overnight and is ready for spinning in a standard cellulose acetate dry spinning cell. Yarn of 100 denier is obtained by extruding the solution through a spinneret having 16 holes of 0.08 mm. diameter, at a head temperature of 58 (3., cell temperature of C., pressure of 250 lbs. per q. in., and a wind-up speed of 200 yds./min. When tested immediate 0 yarn is insoluble in acetone, and after immersion of 21, 32, and

chloroethane at 60 'ing at 150 C. for 3- follows.

I prepared by the tert.

- acetic acid, 7,565 par butyl alcohol process 5.25% S102 and 0.02%

' up speeds of 210 yarn by the use '5 90 minutes at 165 C., the yarn loop tenacities of 1.20, 0.75, wet, and loop elongations respectively. The heated factor, determined as described later, of 1.5. After immersion in tetra- C. for 15 minutes, the heated yarn has dry, wet, and loop tenacities of 1.00, 0.68, and 0.87 g./d'. with corresponding elongations of 34, 44, and32%.' Yarn of substantially being heatedfor has dry, wet, and and 1.0 g./d. and d y,

yarn has a swelling the same physical properties is obtained by heat- 17 hours or for a few seconds at 255 C.

The swelling factor of cellulose acetate yarn in tetrachloroethane at 60 C. is determined as A tuft or small skein of fibers is weighed, immersed in tetrachloroethane at 60. C.i1 C-. for 15 minutes, and centrifuged for 2 minutes in a 5" basket at approximately 3,000 R. P- M. and weighed again. The weight of tetrachloroethane absorbed per gram of yarn is designated as the swelling factor.

Example II This example illustrates the preparation of tetrachloroethane-resistant cellulose acetate yarn by the use of a butyl acid 'polysilicate of low degree of esterification and low molecular weight -butyl aleoholprocess.

A spinning solution is prepared by placing acetate of 54.5% combined ts of an acetone solution te prepared by. the tert.- procedure A) containing 2,486 parts of cellulose of butyl acid polysilic an enclosed paddle-type mixparts of acetone in er, and stirring eight hours at a temperature of about C. After mixing, the solution contains 27.3% acetate filter pack. Yarn of 75 denier was spun through a spinneret containing 24 holes of 0.05 mm. diameter under the following conditions: head temperature 60 C., cell temperature 90 C., pressures of 270-595 lbs; per sq. in., and winduble in acetone and hot tetrachloroethane immediately after being spun. However, byheating it for 90 minutes at 168 which is highly resistant to tetrachloroethane at 60 C. It has a swelling factor of 1.0. The physical properties of the yarn before heating are: dry, wet, and loop tenacities of 0.98, 0.65, and 0.92 g. /d. with corresponding elongations of 15.5, 25.2, and 14.3%. Yarn heated for 90 minutes at 168 C. and immersed in tetrachloroethane at 60 physical properties: dry, wet, and loop tenacities of 0.86, 0.66, and 0.78 g./ d. and corresponding elongations of 23.8, 33.5, and 22.5%, respectively.

Example III This example illustrates the preparation of a polysilicic acid ester-modifiedscellulose acetate of a partially esterified ethyl polysilicate of relatively low molecular weight and high degree of esterification.

A solution for spinning into fibers by the electrostatic procedure is prepared by adding to 71.6 parts of a filtered acetone solution containing 14.25 parts of cellulose acetate of 54.5% combined acetic acid, 17.4 parts of an .ethyl alcohol solution of ethyl acid polysilicate containing 12.9% $102. This modifier is one of relatively low molecular weight and high degree of esteri- 'fication prepared as described in procedure C by sulfuric acid, and 1,000.

solids and is filtered through a cellulose -608 yds./min. This yarn is sol- C., yarn is obtained C. for 15 minutes has the following block ri v 6- the partial hydrolysis of tetraethyl silicate. The resulting solution contains 16% cellulose acetate and 2.24% -SiO2 as ethyl .acid polysilicate. This solution is extruded in a fine stream into an electrostatic field to form fibers by a process similar to that described in U. S. Patent No. 1,975,504 to Formhals. The fibers arethen heated 16 hours 'at 150 C. After this heat treatment the individual filaments are not stuck together after immersion in tetrachloroethane at 60 C. for 15 minutes. With this, modifier, the long baking treatment is necessary to obtain satisfactory resistance to tetrachloroethane. If these fiber are baked for only minutes at 150 C., they are highly swollen by tetrachloroethane at 60 C.

Example IV This example illustrates the modification of cellulose acetate yam with 8% of its weight of S102 in the form of a low molecular weight, lowly esterified butyl acid polysilicate.

A spinning solution containing 22.5% cellulose acetate and 2.0% SiOz as butyl acid polysilicate is made as follows: One thousand one hundred twenty-five (1,125) parts of cellulose acetate of 54.5% combined acetic acid is dissolved in 3,450 parts of acetone and 460 parts of a solution of a low molecular weight, low degree of esterification butyl acid polysilicate in acetone-methanol (4:1) containing2l.8% SiOz. The solution is filtered through a regular cellulose acetate filter pack and spun into yarn of about denier, having 40 filaments, under the following condi-' tions: Head temperature 60-615 C., cell temperature 90 C., pressure 175-185 lbs/sq. in., and wind-up speed 300 yds./min. The resulting yarn has the following physical properties: Dry, wet, and loop tenacities of 1.28, 0.9 and 1.50 g./d.'and corresponding elongation-s of 25, 28, and 23%. After this yarn is heated for 90 minutes at C., it has the following physical properties: Dry, wet, and loop tenacities of 1.20, 0.81, and 1.19 g./d. with corresponding elongation of 20, 18, and 19%. The modified yam has a sticking point about l0-20 C. higher than that of unmodified'cellulose acetate.

The sticking point of cellulose ester yarn is determined as follows: Both ends of a six-inch 50 length of the yarn are tied to a weight of 0.7-0.8

. The weighted end of the yarn is passed over a horizontal brass rod held in front of a Maquenne v block and a few millimeters above its top surface, and the looped end passedaround'another 55 rod held similarly on the other side of the block.

The double strand of yarn is held against the surface of the block in diameter. The block is heated at a rate of 5 C. per minute. As the temperature of the ses, the weight is lifted from the yarn every 5 C. At low temperatures the yarn comes immediately off the surface of the block, butat the sticking temperature adheres to the block. According to this test unmodified cellulose acetate 65 yarn sticks at C. It will be understood, of course, that the above examples are merely by way of illustration and that the invention is not limited to the exact agents and conditions set forth therein but is 7 susceptible rather to a wide substitution of materials and variation in the essential conditions.

' Thus, in addition to the ethyl and tert.-butyl acid polysilicates which are specifically described in the examples as modifiers for cellulose acetate,

75 partially esterified silicic acid esters of other alhighly esterified type of by a 200 g. brass weight 3 cm.

' alcohol, normal and isopropanol, n-butanol, and

tert.-amyl alcohol.

The preferred types of polysilicic acid esters for use as modifiers in cellulose acetate yarn are those of relativelylow molecular weight and low degree of esterification. However, other types of polysilicic acid esters will also produce some degree of solvent resistance and also heat resistance. As indicated in Example III, an ethyl acid polysilicate of low molecular Weight and high degree of esterification requires a much longer heating treatment (16 hours at 160 C.) to obtain anacetate yarn insoluble in tetrachloroethane than a butyl acid polysilicate of low degree of esterification and low molecular weight, which requires .only 90 minutes at 160 C. Likewise, polysilicic acid esters of relatively high molecular weight are not as effective as those of low molecular weight.

The preferred amount of partially esterified polysilicic acid ester to be used in modifying cellulose acetate is from 12 to 16 parts (calculated as $102) of modifier for 100 parts of cellulose acetate. On the basis of the amount of S102 in the yarn, this corresponds to from 10.7 to 13.8%. However, amounts of polysilicic acid esters ran ing from parts (calculated as SiOz) per 100 parts of cellulose acetate to as much as 20 parts or more per 100 parts of cellulose acetate may be used. Cellulose acetate yarn modified with amounts of polysilicic acid esters amounting to 5 to parts per 100 are less resistant to solvents than yarns modified with the preferred proportions. Cellulos acetate yarns modified with 20 carbonate for even 1 to 2 hours at a temperature of 0-5 C., the molecular weight of the polyor more parts of partially esterified polysilicic acid (calculated as SiO2) are resistant to solvents, after heat treatment; but may have inferior physical properties due to incompatibility of the modifier with the cellulose acetate. of the modifier will depend on the particular ester being used, an ethyl ester, for example, being more compatible than a butyl or amyl ester, and esters of lowest molecular weight are more compatible than those of higher molecular weight.

The minimum amount of free'mineral acid, e. g. sulfuric acid or hydrochloric acid, which has been found necessary to maintain satisfactory stability is about 0.01 to 0.015% in a spinning solution containing approximately 25% cellulose acetate. The amount of free sulfuric acid carried along with a modifier prepared by the tributyl phosphate extraction of an aqueous silicic acid solution is just sufficient to form this concentration of free acid in a spinning solution containing 10-16 parts of SiOz per 100 parts of cellulose acetate. However, when an aqueous polysilicic acid solution is extracted by tert.-butyl alcohol, a larger amount of sulfuric acid is carried along with the butyl acid polysilicate. With this type of modifier the acidity must be reduced so that the cellulose acetate spinning solution to which it is added wil1 not contain more than 0.015% sulfuric acid. As described in Procedure A above, treatment with solid barium carbonate is satisfactory. In this treatment care must be taken that the acetone solution of butyl acid polysilicate is allowed to stand in the presence of the barium carbonate no longer than is necessary to reduce the free acid to the desired limits since polysilicic acid esters polymerize quite rapidly at a pH greater than 3.0. If an acetone solution is allowed to stand in the presence of excess barium The compatibility g lulose acetate yarns modified with partially estersilicic acid ester will increase sufficiently to make it inferior as a. modifier for cellulose acetate. The presence of more than 0.015% sulfuric acid in the cellulose acetate spinning solution will improve the filterability and spinnability of they solution but will have a deleterious effect on the physical properties of the yarn obtained from such solutions.

The preferred heat treatment for rendering silicic acid ester-modified cellulose acetate yarn insoluble in'tetrachloroethane is about 90 minutes at 160-170 C. However, temperatures and times varying from these may be used. For example, the yarn prepared as described in Example I is somewhat resistant to tetrachloroethane at 60 C. even without being heated. When heated 15 minutes at 120 C. it has a swelling factor .of 7.2, and when heated for 90 When heated for 90 minutes at 150 C. it has a swelling factor of 2.2, and after 13 hours at 155 C. it has a swelling factor of 0.8, which is considered ,to be good resistance to tetrachloroethane at 60 C. When heated 90 minutes at 165 C., this yarn has a swelling factor of 1.5, and this is also considered satisfactory resistance. The same yarn heated for a few seconds at 255 C. has a swelL ing factor of 0.97.

Cellulose acetate yarn modified with polysilicic acid esters may be prepared in the form of continuous filament yarn, or it may be made into staple fibers by customary procedures, depending on the particular use for which the yarn is desired.

Likewise, polysilicic acid ester-modified cellulose acetate yarn may contain other modifiers in addition to the partially esterified polysilicic acid. For example, a delusterant may also be incorporated in the yarn.

Cellulose organic acid'ester yarns such as celified polysilicic acids are particularly useful in the manufacture of fabrics which are to be subjected to comparatively drastic heating treatments or exposed to organic solvents.

We claim:

1, A process which comprises spinnin fibers from an organic solvent'solution comprising essentially an organic acid ester of cellulose, from 5 to 20 parts by weight of a low molecular weight partial ester of a polysilicic acid and a monohydric aliphatic alcohol having from one to six carbon atoms, per 100 parts of the cellulose ester,

and from 0.01 to 0.015% by weight of a mineral acid, and heat treating the fibers for a period of time sufficient to render the same substantially insoluble in tetrachloroethane.

2. A process which comprises spinning fibers from'an organic solvent solution comprising essentially an organic acid ester of cellulose, from 12 to 16 parts by weight of a low molecular weight partial ester of a polysilicic acid and a monohydric aliphatic alcohol having from one to six carbon atoms, perparts of the cellulose ester, and from 0.01 to 0.015% by weight ofa mineral acid,and heat treating the fibers for a period of time sufficient to render the same substantially insoluble in tetrachloroethane.

3. A process which comprises spinning fibers from an organic solvent solution comprising essentially an organic acid ester of cellulose, from 12 to 16 parts by weight of a low molecular weight partial ester of a polysilicic acid and a monominutes at it has a swelling factor of 3.1.

hydric aliphatic alcohol having from one to six carbon atoms, per 100 parts of the cellulose ester,

. and from 0.01 to 0.015% by weight of a mineral acid, the ratio of ester groups to silicon atoms in said partial ester being less than 0.6 to 1.0, and heat treating the fibers for a period of time sufiicient to render the same substantially insoluble in tetrachloroethane.

4. A process which comprises spinning fibers from an organic solvent solution comprising essentially cellulose acetate, from 12 to 16 parts by weight of a low molecular weight partial ester of a polysilicic acid and a monohydric aliphatic alcohol having from one to six carbon atoms, per 100 parts of cellulose acetate, and from 0.01 to 0.015% by weight of a mineral acid, and heat treating the fibers for a period of time suflicient to render the same substantially insoluble in tetrachloroethane.

5. A process which comprises spinning fibers from an organic solvent solution comprising 5- sentially cellulose acetate, from 12 to 16 parts by weight oi-a low molecular weight partial ester of a polysilicic acid and a monohydric aliphatic alcohol having from one to six carbon atoms, per 100 parts of cellulose acetate, and from 0.01 to 0.015% by weight of a mineral acid, the ratio of ester groups to silicon atoms in said partial ester being less than 0.6 to 1.0, and heat treating the fibers for a period of time sufiicient to render the same substantially insoluble in tetrachloroethane.

6. A process which comprises spinning fibers from an organic solvent solution comprising essentially cellulose acetate, from 12 to 16 parts by weight of a low molecular weight partial ester of a polysilicic acid and a monohydricaliphatic alcohol having from one to six carbon atoms, per lOOparts of cellulose acetate, and from 0.01 to 0.015% by weight of sulfuric acid, the ratio of ester groups to silicon atoms in said partial ester being less than 0.6 to 1.0, and heat treating the fibers for a period of time sufiicient to render the same substantially insoluble in tetrachloroethane.

7. A process which comprisesincorporating in an acetone solution of cellulose acetate from 5 to 20 parts by weight of a low molecular weight partial ester of a polysilicic acid and a monohydric aliphatic alcohol having from one to six carbon atoms, per 100 parts of cellulose acetate, and from 0.01 to 0.015% of a mineral acid, dry spinning the solution to form fibers and heat treating the fibers for a period of time sumcient to render the same substantially insoluble in tetrachloroethane.

8. A process which comprises incorporating in an acetone solution of cellulose acetate from 5 to 20 parts by weight of low molecular weight partial ester of a polysilicic acid and ammohydric aliphatic alcohol having from one to six carbon atoms, per 100 parts of cellulose acetate, and from 0.01 to 0.015% of a mineral acid, the ratio of ester groups to silicon atoms in said partial ester being less than 0.6 to 1.0, dry spinning the solution to form fibers and heat treating the fibers tor a period of time sumcient to render the same substantially insoluble in tetrachloroethane.

' 9. A process which comprises incorporating in 10 an acetone solution of cellulose acetate from 12 to 16 parts by weight of a low molecular weight partial ester of a polysilicic acid and a monohydric aliphatic alcohol having from one to six carbon atoms, per parts of cellulose acetate, and from 0.01 to 0.015% of a.mineral acid, the ratio of ester groups to silicon atoms in said partial ester being less than 0.6 to 1.0, dry spinning the solution to form fibers and heat treating the fibers for a period of time sufilcient to render the same substantially insoluble in tetrachloroethane.

' 10. A process which comprises incorporating in an acetone solution of cellulose acetate from 12 to 16 parts by weight of a. low molecular weight partial ester of a polysilicic acid and a monohy- 'dric aliphatic alcohol having from one to six carbon atoms, per 100 parts of cellulose acetate,

and about 0.015% of sulfuric acid, dry spinning the solution to form fibers and heating the fibers from one to two hours at a temperature of from to 0., whereby to render the fibers substantially insoluble in tetrachloroethane.

11. A process which comprises incorporating in an acetone solution of cellulose acetate from 12 to 16 parts by weight of a low molecular weight partial ester of a polysilicic acid and a monohydri'c aliphatic alcohol having from one to six carbon atoms, per 100 parts of cellulose acetate, and about 0.015% of sulfuric acid, dry spinning the solution to form fibers and heating the fibers for about 90 minutes at a temperature of from 160 to 170 0., whereby to render the fibers substantially insoluble in tetrachloroethane.

12. A process according to claim 9 wherein the partial ester is a butyl acid polysilicate.

13. A process according to claim 10 wherein the partial ester is a butyl acid polysilicate.

14. As a new article of manufacture fibers comprised essentially of cellulose acetate and a low molecular weight partial ester of a polysilicic acid and a monohydric aliphatic alcohol having from one to, six carbon atoms, said fibers having been rendered substantially insoluble in tetrachlorethane at 60 C. by subjecting them to a temperature offrom 150 C. to 255 C. for a period of time varying from a few seconds at the higher temperature to asmuch as 17 hours at the lower temperature.

15. As a new article of manufacture fibers comprised essentially of cellulose acetate and from 10.7 to 13.8% by weight (calculated as SlOz) of a low molecular weight partial ester of a polysilicic acid and. a monohydric aliphatic alcohol having from one to six carbon atoms, the ratio of ester groups to silicon atoms in said partial ester being less than 0.6 to 1.0, said fibers having been rendered substantially insoluble in organic solvents by subjecting them to a temperature of ROBERT E. FQTHERGHL. JULIAN w. HILL, ALERED A. JOHNSON. 

